1. Field of the Invention
The present invention relates to a method for manufacturing a color cathode ray tube, such as is used for a television or a computer display. More particularly, it relates to a method for manufacturing a color cathode ray tube in which a shadow mask is fixed by welding it to a mask frame while applying a tension force to the shadow mask and a compression force to the mask frame.
2. Description of the Prior Art
In recent years, ever flatter color cathode ray tube front panels have brought about ever flatter shadow masks. However, as shadow masks become flatter, they cannot be kept flat only by supporting the main body of the shadow mask with a frame, as in conventional shadow masks. In addition, when the main body of the shadow mask is supported only by a frame, the shadow mask vibrates easily due to vibrations from the outside, which adversely affects the display screen of the color cathode ray tube. Therefare, the shadow mask is stretched and fixed to the frame with a constant tension applied to the shadow mask.
On the other hand, also regarding the doming effect, in which the thermal expansion, caused by the impact of the electron beam on the shadow mask, deforms the shadow mask, flatter shadow masks lead to a larger displacement of the electron beam due to the doming, especially in the vicinity of both ends of the screen. Thus, when stretching and fixing the shadow mask as described above, the largest practical level of tension, close to the elastic limit, is applied to the shadow mask, in order to absorb the thermal expansion due to the impact of the electron beam.
Stretching and fixing the shadow mask like this can prevent vibrations of the shadow mask due to outer vibrations, and misalignments between the relative positions of apertures for passing electron beams in the shadow mask and phosphor dots on a phosphor screen, even when the temperature of the shadow mask rises.
Thusly stretched shadow masks are called "tension-type shadow masks". Among the different kinds of tension-type shadow masks, there is the aperture grille-type shadow mask, in which a plurality of slender members are laid across the mask frame, the slot-type shadow mask, in which a plurality of substantially rectangular apertures for passing electron beams are formed in a flat panel, and the dot-type shadow mask, in which a plurality of round apertures for passing electron beams are formed in a flat panel.
Also, shadow masks can be stretched with a one-dimensional tension system or a two-dimensional tension system. "One-dimensional tension system" refers to systems that apply a tension only to the vertical shadow mask direction (parallel to the short sides of the shadow mask), while "two-dimensional tension system" refers to systems that apply a tension both to the vertical and horizontal direction. The one-dimensional tension system is used for aperture grille-type shadow masks, and both systems are used for slot-type and dot-type shadow masks.
With regard to methods for manufacturing a color cathode ray tube in which a shadow mask fixed by welding it to a mask frame while a tension force is applied to the shadow mask and a compression force is applied to the mask frame, several welding methods have been suggested. Publication of Japanese Unexamined Patent Application (Tokkai) No. Sho 64-84540 discloses a method for welding the central portion in the width direction of the upper side of the mask frame along each side of the mask frame.
Publication of Japanese Unexamined Patent Application (Tokkai) No. Hei 4-22042 discloses a method for laser welding the central portion in the width direction of an upper side of the mask frame along each side of the mask frame. In addition, Publication of Japanese Unexamined Patent Application (Tokkai) No. Sho 62-232832 discloses a method for moving a welding electrode on a support member (i.e. mask frame).
However, these conventional methods for manufacturing a color cathode ray tube lead to the following problems: (1)The mask frame is rectangular, and when a uniform compression force is applied throughout one entire side, the central portions of the sides of the mask frame warp inward, and the upper edge of the frame tilts inward since the cross section of the frame is usually L-shaped. Therefore, welding the upper surface of the frame requires a complex control of moving the position of the welding point according along the flexure curve of the frame. Furthermore, when the upper side of the frame is tilting inward, it is difficult to bring the shadow mask into close contact with the upper side of the frame (the upper surface of the upright portion of the L-shaped portion) for the welding step. (2) Since the distance between the shadow mask and the phosphor screen (q-value) requires strict control, precision of the position of welding surface where the shadow mask is fixed is important. When for some reason a failure of the shadow mask occurs, the defect shadow mask is sometimes torn off for replacement and repair. If the remaining surfaces of weld nuggets are abraded so as to weld the shadow mask on again, the welding surface is lowered by the abraded amount, resulting in a substantial deterioration of the precision for the position of the welding surfaces. Accordingly, the initial q-value cannot be ensured. In other words, a slight failure of the shadow mask can lead to a total failure of the shadow mask structure including the mask frame.